1. Field of the Invention
This invention relates to a control device for a vehicular drive system having an electric differential portion, incorporating a differential mechanism operative to perform a differential action, and a transmission portion disposed in a power transmitting path between the differential portion and drive wheels and, more particularly, to a technology of controlling a shifting action in the transmission portion in response to a shift demand for the shifting.
2. Description of the Related Art
A control device for a vehicular drive system has heretofore been well known as including a differential portion, having a differential mechanism operative to distribute an engine output to a first electric motor and a power transmitting member, and a transmission portion i.e., shifting portion disposed in a power transmitting path between the differential portion and drive wheels. The differential portion includes a first element, connected to the engine, a second element connected to the first element and the first electric motor, and a third element connected to the power transmitting member. With such a structure, the differential portion performs a differential action with a speed ratio being continuously varied to function as a continuously variable transmission.
Meanwhile, Patent Publication (Japanese Patent Application Publication No. 9-37410) discloses a technology related to a vehicular drive control device, having a continuously variable transmission, which varies a speed ratio of the continuously variable transmission with a variation in a vehicle speed. This allows an engine speed to be maintained at a fixed level regardless of the variation in vehicle speed, i.e., in other words, regardless of a variation in an output rotational speed of the continuously variable transmission. In addition, other technologies have heretofore been known as disclosed in Japanese Patent No. 3526955 and Japanese Patent Application Publication No. 2002-243031.
Even with a control device for a vehicular drive system having a differential portion and a transmission portion, the shifting is initiated in the differential portion in accordance with the shift in the transmission portion with a view to rendering the engine operative in an operating range with high efficiency. This makes it possible to maintain the engine speed at a nearly fixed level on a stage before and after the shifting regardless of a variation in output rotational speed of the differential portion due to the shift in the transmission portion.
However, during the operation of the control device for the vehicular drive system with such a structure mentioned above, the shifting is encountered with an issue. That is, depending on the relationship between the output rotational speed of the transmission portion and the engine speed, the first electric motor (second element) rotates at a speed, determined based on the relationship on mutual relative rotational speeds of the first to third elements of the differential portion, which results in a high rotation with a resultant drop in durability of these component parts. Besides, pinion gears, forming the differential mechanism, have rotation speeds (in another point of view, a difference between, for instance, a rotational speed of the engine (first element) and a rotational speed of the power transmitting member (third element)) falling in a high rotation range with a resultant drop in durability of the pinion gears (such as, for instance, pinion needle bearings and bushes, etc.). That is, depending on a shift demand for the automatic transmission portion, i.e., a speed ratio required for the transmission portion, there has been likely that the first electric motor rotates at a high speed and the pinion gears are caused to rotate at the high speed. No research and study work has heretofore been made in the related art with a view to addressing the occurrence of the high rotation of those component parts and the issues remained publicly unknown.
FIG. 12 is a well-known collinear chart representing rotational speeds of respective rotary elements forming the differential portion. The collinear chart shows examples of a variation in rotational speeds of the respective rotary elements with an up-shift executed in the transmission portion together with the relationship associated with the output rotational speed of the transmission portion. In FIG. 12, reference “ENG” represents the rotational speed of the first rotary element (first element) connected to the engine; “M1” the rotational speed of the second rotary element (second element) connected to the first electric motor; “M2” the rotational speed of the third rotary element (third element) connected to the power transmitting member and the second electric motor; and “OUTPUT” the output rotational speed of the transmission portion. In addition, respective straight lines, related to the differential portion, represent relative motion relationships in rotational speed among the respective rotary elements. Solid lines indicate the relative motion relationships prior to the execution of an up-shift action and broken lines indicate the relative motion relationships subsequent to the up-shift action.
As the up-shift executed with a decrease in the rotational speed “M2” as shown in FIG. 12, the rotational speed “M1” of the second element is raised so as to maintain the rotational speed “ENG” of the first element at a nearly fixed level. During such an up-shift, if the output rotational speed of the transmission portion remains in a relatively low state with the engine speed remained in a relatively high state, there has been likelihood of the first element having an increasing rotational speed so that the first electric motor rotates at a high speed. Besides, this results in a relatively increased difference in rotational speed between the engine speed and the power transmitting member (second electric motor), causing a probability to occur with the pinion gears, forming the differential portion, to rotate at high speeds.
Although the foregoing has been described with reference to the sup-shift operation of the transmission portion, it is needless to say that the pinion gears are liable to rotate at the high speed even when a downshift operation is effectuated in the transmission portion. In this case, the rotational speed of the first electric motor merely lays in a negative phase and, similarly, there has been likelihood of the first electric motor caused to rotate at a high speed. In addition, the foregoing has been exemplarily described with reference to the shifting initiated by the control device of the vehicular drive system in which a shifting control is performed to keep the engine speed at the nearly fixed level in accordance with the shifting in the transmission portion on a stage before and after the shifting. However, the particular arrangements described are meant to be illustrative only and the present issues are not limited to such a shifting control. It is of course to be appreciated that the present issues are encountered even when, for instance, the engine speed is varied on a stage before and after the shifting action.